Goal:
    x2 : V.t  n2 : Z  y2 : V.t
  ============================
   ((x2, n2, y2) = (x2, n2, y2))


DEBUG:Going to execute:
match goal with
           | |- ?X1 -> ?X2 => intro
           | |- ~ _ => unfold not at 1; intro
           end